Real agl ?

[Airborne_Aircrew]

So be shocked:

I am shocked that your GPS is reporting 66.9m, yes meters, (219 feet), below sea level at the same time as the "absolute altitude" is reporting 66.93m below sea level. I can go to my driveway right now and get in my truck with my phone and have 4 different GPS' report 648' AMSL +/- 10 feet. Your data is erroneous - it happens, don't worry.

 

[Meta4]

I am shocked that your GPS is reporting 66.9m, yes meters, (219 feet), below sea level

My GPS isn't giving that information at all, DJI is using barometer data but mis-labelling it as GPS data.
This was explained in the link in post #25.

Your data is erroneous - it happens, don't worry.

It happens all the time with most DJI drones except the Mavic 2 if it's using recent firmware.

 

[Airborne_Aircrew]

Maybe I wasn't clear on that point. DJI does put barometrically-derived altitude AMSL into the EXIF dat under some older and some current firmware versions. That's not a guess - it's a demonstrable fact. As for being useful to an aviator - I'm not aware of any aviators using EXIF for navigation purposes.

That's not the point I was making so your response is a little "avoiding". There is, simply, no way of determining actual altitude other than QFE from the current barometric pressure. The ISA is a theoretical model that bases it's assumptions on a fixed set of data points that will rarely reflect the real world. Therefore, it is impossible for DJI to put "barometrically derived" AMSL into anything, period - because it can't be done. My post above demonstrates that the data is erroneous because it demonstrates that GPS data and "barometric data" coincide when they clearly can't.

You're final "dig' indicates that you aren't, and have never been, an aviator because it is rather silly...

AMSL is only really important to airline pilots and those that use airways and other such high altitude users. It's the way they maintain separation of altitude so they can pass through the exact same geographic location at the same time while guaranteeing vertical separation - thus not crashing into each other.

Helicopters, (my experience), live in the lower altitudes, along with drones, and while we were quite familiar with flight levels and airways we avoided them because they made flying boring. Much of my flying was done below 100'... probably lower than you fly you drone other than take-off and landing.

You cannot accurately derive AMSL from current barometric pressure. I'm trying to help you learn this but you are being very resistant. Listen.

 

[Airborne_Aircrew]

My GPS isn't giving that information at all, DJI is using barometer data but mis-labelling it as GPS data.
This was explained in the link in post #25.

It happens all the time with most DJI drones except the Mavic 2 if it's using recent firmware.

In post #25 you said:-

This demonstrates, as usual, that the barometric altitude is unreliable, both in absolute terms (around a 400 ft error), but also that the error in relative barometric altitude, which is expected to be linear with relative altitude, can be quite noticeable at 400 ft above takeoff – in this case around 4 meters (13 ft).

This is my point... AMSL _CANNOT_ be determined by the current barometric pressure locally...

It happens all the time with most DJI drones except the Mavic 2 if it's using recent firmware.

Why do I think you own a Mavic 2 and believe some things that are completely accurate?

 

[Meta4]

Why do I think you own a Mavic 2 and believe some things that are completely accurate?

I have no idea ... or a Mavic 2.
Here are a couple more submarines for you that demonstrate that it happens all the time.
If you shoot a lot around sea level, you'd notice.

 

[sar104]

That's not the point I was making so your response is a little "avoiding". There is, simply, no way of determining actual altitude other than QFE from the current barometric pressure. The ISA is a theoretical model that bases it's assumptions on a fixed set of data points that will rarely reflect the real world. Therefore, it is impossible for DJI to put "barometrically derived" AMSL into anything, period - because it can't be done.

I have no idea why you are still arguing. Are you still disputing that the FC uses barometric pressure and a standard atmospheric model to derive absolute altitude for the EXIF data, or are you just trying to deflect from your previous assertion that it didn't without admitting that you were wrong? No one is arguing that it gives an accurate altitude, and @Meta4 clearly demonstrated that it doesn't. But that doesn't change the fact that DJI did exactly that throughout the range until they modified the Mavic 2 firmware. And of course it can be done - barometric pressure is a single-valued function of altitude in the standard atmospheric model, and so given a barometric pressure it is trivial to calculate an altitude.

My post above demonstrates that the data is erroneous because it demonstrates that GPS data and "barometric data" coincide when they clearly can't.

No - your post above demonstrates that you either didn't read or completely misunderstood the explanation in the link that I provided earlier - the EXIF field labeled GPS altitude is not GPS altitude - that was a large part of the problem. Both absolute and GPS altitude fields were populated with the same barometrically-derived altitude AMSL. That's why they are exactly the same.

You're final "dig' indicates that you aren't, and have never been, an aviator because it is rather silly...

No - what was silly was your attempt to argue that DJI wouldn't do that because it isn't useful to aviators - hence my point.

AMSL is only really important to airline pilots and those that use airways and other such high altitude users. It's the way they maintain separation of altitude so they can pass through the exact same geographic location at the same time while guaranteeing vertical separation - thus not crashing into each other.

No - that would be FL, but don't let me distract you from trying to change the subject at every possible opportunity.

Helicopters, (my experience), live in the lower altitudes, along with drones, and while we were quite familiar with flight levels and airways we avoided them because they made flying boring. Much of my flying was done below 100'... probably lower than you fly you drone other than take-off and landing.

That's fascinating.

You cannot accurately derive AMSL from current barometric pressure. I'm trying to help you learn this but you are being very resistant. Listen.

The irony is almost painful, since that is what both @Meta4 and I have been trying to explain, over and over again. The barometrically-derived altitude is completely unreliable but nevertheless that's what goes into the EXIF file.

 

[Meta4]

AMSL can never be derived from the local atmospheric pressure at any place but on a beach

You are correct, AMSL can't accurately be derived that way.
But that's what DJI does for their misnamed GPS height that is only seen in Exif data.
And that's why it is so inaccurate.

Anyone arguing the use of that model in any actual flight situation is being, well... "abrasive".

No-one has suggested that it's used in any actual flight situation.
The incorrect data only shows up in Exif info for photographs and you won't see it until you put your images on a computer.

 

[sar104]

I've been told I'm a naughty boy because I am "abrasive" and therefore, in the interest of no longer being "abrasive", (something it appears that no-one else in this conversation has been accused of), I will withdraw.

My assertion that AMSL cannot be derived from local pressure data is, apparently, wrong because someone else mentions ISA in a cocky and somewhat superior fashion. It seems that that automagically negates my many hundreds of hours of actual flight time in real, (Military), aircraft over several years.

You don't get to weasel out with that kind of post either. Unfortunately your hundreds of hours of flight taught you nothing about physics, and your continued false appeal to that authority is both pointless and tiresome. The obvious and undisputed fact that barometric pressure doesn't yield an accurate altitude doesn't change: (1) that barometric pressure nevertheless allows computation of an altitude (albeit not an accurate one) and (2) barometric is what DJI has used to generate the absolute and (misnamed) GPS altitudes in EXIF files.

 

[dougiefresh27]

400' limit where I fly is interesting because I can take off from a 300' high bluff ascend to 400' and fly over the river. That would put the drone 700' above the river. But as long as I'm no higher than 400' above my take off point I should be ok.

 

[Capt KO]

400' limit where I fly is interesting because I can take off from a 300' high bluff ascend to 400' and fly over the river. That would put the drone 700' above the river. But as long as I'm no higher than 400' above my take off point I should be ok.

No, Once you are 400 feet out, you will need to descend 300ft, to stay 400 feet above the river. The same as if you were taking off from the river bank. Good luck.

 

[The Editor]

400' limit where I fly is interesting because I can take off from a 300' high bluff ascend to 400' and fly over the river. That would put the drone 700' above the river. But as long as I'm no higher than 400' above my take off point I should be ok.

Nope.

 

[Bob4131]

So , thank you all.
There is no way for the pilot to know the actual AGL of the AC ...
True ?

As discussed above, the drone altitude measurement above the take off spot is not perfect. Not sure your question, but I do the following when flying in non-level terrain.
I use the app "My Elevation" to understand the height of the hills and depth of the valleys around my takeoff location. If I am in a valley and the hill is 350ft high, I can legally (US) fly 400 ft above the highest point of the hill, which would be 750 feet higher than the takeoff location. But going to that point, I need to stay within 400 ft of the ground below the UAS.
So I don't know the exact height agl, but this app provides enough information to remain legal.

 

[Bob4131]

No, Once you are 400 feet out, you will need to descend 300ft, to stay 400 feet above the river. The same as if you were taking off from the river bank. Good luck.

This is different for hobbyist vs 107 flight. The hobbyist is required to never be more than 400' above the ground. As the UAS leaves the bluff, it is required to drop to 400' above ground level constantly.
Under 107 rules, there is a 400' radius rule.

 

[sar104]

This is different for hobbyist vs 107 flight. The hobbyist is required to never be more than 400' above the ground. As the UAS leaves the bluff, it is required to drop to 400' above ground level constantly.
Under 107 rules, there is a 400' radius rule.

There is no 400 ft radius rule for terrain - only for structures.

 

[Bob4131]

There is no 400 ft radius rule for terrain - only for structures.

Correct - I just wanted to address the hobbyist side of the rules which the OP is probably flying under.

 

[sar104]

Correct - I just wanted to address the hobbyist side of the rules which the OP is probably flying under.

Right - you are completely correct regarding hobby flight. And it's the same for Part 107.

 

[Meta4]

400' limit where I fly is interesting because I can take off from a 300' high bluff ascend to 400' and fly over the river. That would put the drone 700' above the river. But as long as I'm no higher than 400' above my take off point I should be ok.

Using that logic, one of these flyers is breaking the rules and one isn't even though they are flying their drones at the same place.

 

[Bob4131]

Right - you are completely correct regarding hobby flight. And it's the same for Part 107.

OK, I have to tell a story ?. I was in Colorado and saw a UAS operator fly over the edge of a 500' cliff. After his flight, I was chatting and he brought up if I thought his flight was legal. I said no. He said he was flying 107 that day but I said the 400' radius only applied to structures. He said true and pointed to his "structure". It was a restroom between the cliff edge and parking lot perhaps 150' away. His comment was that he "often can find a structure to use". I am not sure that fulfills the FAA definition of a structure - it certainly was not the "tower inspection" they were trying to allow for 107 operators, but it was in the back of my mind as I wrote the above. ?
I really just wanted to highlight that 400' above ground level immediately below the drone is the rule in the US.

 

[sar104]

OK, I have to tell a story ?. I was in Colorado and saw a UAS operator fly over the edge of a 500' cliff. After his flight, I was chatting and he brought up if I thought his flight was legal. I said no. He said he was flying 107 that day but I said the 400' radius only applied to structures. He said true and pointed to his "structure". It was a restroom between the cliff edge and parking lot perhaps 150' away. His comment was that he "often can find a structure to use". I am not sure that fulfills the FAA definition of a structure - it certainly was not the "tower inspection" they were trying to allow for 107 operators, but it was in the back of my mind as I wrote the above. ?
I really just wanted to highlight that 400' above ground level immediately below the drone is the rule in the US.

That's an imaginative explanation. It's also, technically, correct.

 

[Thomas B]

That's an imaginative explanation. It's also, technically, correct.

Sometimes this type of thing is reasonable... within close proximity of the cliffs.
Example: TO on top of the Mogollon Rim in central AZ, rise to 10 feet above TO.... then fly out 50 feet and my drone is about 2000’ above ground below it as the Rim is a giant 2000’
Escarpment stretching 250-300 miles across central AZ.